This invention relates to the manufacture of freeform-fabricated core composite articles, and more specifically, to a method of manufacturing composite articles wherein the core is fabricated in a freeform-fabrication machine, which exactly matches the article""s geometry and dimensional specifications. This method eliminates the need to construct rigid molds, sculpted plugs, sculpted cores or preformers.
Composite articles are used to produce items that must be lightweight but very strong and very durable. For example, composite fabricated articles have been embraced by the aerospace industry in recent years, where advanced composites have been used to build airframe structures such as wing skins, fuselage sections and many other internal structures and components. In fact, several newly designed general aviation aircraft and most new military aircraft are comprised largely of composite materials. While expensive to develop and perfect, composite-framed aircrafts are lighter, stronger, and more durable than metal-framed aircrafts. Advanced composites are also used to produce a wide-range of other products, such as top-of-the-line sporting goods and racing bikes, but their use in these products are normally limited to applications where the superior properties of the composite (i.e.: high strength to a weight ratio, resistance to corrosion and ease of repairability) is of greater importance than the relatively higher cost of the composite and overall greater cost of manufacturing with composite materials when compared to traditional materials.
Unfortunately, the higher cost of composite product development and manufacturing, compared to manufacturing with traditional materials, is not sufficiently offset by their desirable qualities to be a practical alternative for most mass production durable goods industries.
Much of the higher cost attributed to manufacturing products in advanced composites is due, in large part, to the current method of production that relies upon the use of part specific rigid molds or sculpted plugs. When the product involves complex geometry or when design changes are likely during development and testing, the tooling cost alone can render all but the xe2x80x9chighest valuedxe2x80x9d products uneconomical :using current composite manufacturing methods. Many products could benefit, however, from the added strength, better durability and lighter weight that is realized using composite materials if a method were available that would reduce the cost to manufacture the article using composite materials.
Composite articles typically comprise a core, a plurality of composite skins and an adhesive film. The core is either formed in the basic shape of the desired composite article or is held to a desired shape by a rigid mold. The composite skin comprises one or more thin layers of material used to cover the exterior of the core and is comprised of one or more laminates of impregnated fiber reinforced composite material having a fiber reinforcement such as graphic, aramid or fiberglass fibers disposed in a binding matrix such as epoxy, phenolic or other similar resinous material. Most of the composite articles produced with present methods require a multi-layered composite skin in order to provide the compressive reinforcement required for the composite article. The adhesive film holds the composite skins in place against the exterior surface of the core during the fabrication process and promotes the bond between the composite skins and the core to produce a composite article with the desired physical properties and shape. In certain applications, reinforcing core sheeting material is xe2x80x9csandwichedxe2x80x9d in between the composite skins, thus creating a sheet of reinforced core composite material, which can then be used in place of other sheeting materials. The composite skins are positioned directly against the mold or sculpted core by hand to create an assembly referred to as a xe2x80x9clay-upxe2x80x9d. In a few industries, the manual lay-up process has been replaced by the use of robotic automated tow placement machines.
The most common method of manufacturing composite articles n involves the use of a vacuum bag assembly wherein an impervious membrane or xe2x80x9cvacuum bagxe2x80x9d is employed to consolidate the plurality of composite skins with the core to ensure proper adhesion thereof. The lay-up is placed inside a vacuum bag assembly where vacuum pressure consolidates the composite skins with the core. To improve surface finish, the lay-up assembly is sometimes positioned directly against female molds before being placed in a vacuum bag. Then the lay-up assembly, inside the vacuum bag assembly, is xe2x80x9ccuredxe2x80x9d by placing it under constant vacuum pressure within an autoclave, thereby subjecting the lay-up assembly to a higher than ambient temperature and/or higher than ambient atmospheric, pressure for a prescribed period of time. Some binding matrix and composite material combinations are able to cure at ambient atmospheric temperature and pressure and thus may be oven or dry air cured.
Rigid molds are most commonly formed using a xe2x80x9cpart positivexe2x80x9d as a pattern made from plaster, silicone or other similar material. The resultant negative mold is then used to form a more durable mold. When a 3D CAD design of the article is available, negative molds may also be milled or machine sculpted from a block of solid material. Rigid production molds are formed using various materials and methods depending upon the industry and the application. Rigid molds are commonly constructed from steel, cured composite, wood, ceramic or other rigid material.
Aerospace advanced composite parts, where quality and repeatability are vitally important, are commonly fabricated by first creating xe2x80x9couter skinsxe2x80x9d by shaping composite skins against rigid part specific molds, hollow structures, or tubes made from shaped plugs, or by xe2x80x9cbuild-upsxe2x80x9d that are fabricated using multiple layers of the composite skins over a mandrel. In some instances, such as in helicopter or propeller blades, the build-up is fabricated by placing multiple layers of the composite skins over shaped wood cores where the wood cores are sometimes referred to as xe2x80x9cpre-formersxe2x80x9d. To achieve the desired level of repeatability necessary to meet government or customer requirements, rigid mold produced composite articles represent the largest portion of the current market. For this process, a composite skin is produced first with the focus of capturing the external geometry of the article. Internal structures are mated to the skin using adhesives. The rigid mold itself is often used as a fixture to hold the various composite and non-composite component parts in place during the bonding and curing process. The resulting complex bonded assemblies, which due to the nature of their assembly, may suffer de-bonding or delamination over time. Take the example of an aircraft wing. In a simple design, the upper and lower wing surfaces are formed by xe2x80x9claying upxe2x80x9d composite skins over a sculpted foam plug. The foam material adds compressive strength to the wing, but precludes the use of the internal space to house fuel or other components. In more complex aircraft, aircraft wings must carry fuel, landing gear, electronics, electrical wiring, hydraulic lines, fuel pumps and control system components to name a few. Individual composite components mated in a wing assembly may total eighty or more separate components. Currently, composite product designers are confronted with the potential hazard of de-bonding and dissimilar materials corrosion as well as the challenges of accurate placement-of parts within the assembly.
As an alternative to rigid molds, the part positive can be produced as a composite article using a sculpted plug as a core. Sculpted plugs are formed in various ways. Computer controlled routers, CNC mills and lathes are often employed to create an accurately shaped plug. Hand shaping, using spaced sectional patterns as a guide are still common in certain industries. Plugs are also shaped using a xe2x80x9chot wirexe2x80x9d device as a cutting tool to sculpt polyurethane, polystyrene or divinycell foam blocks.
The advantages of using freeform-fabrication technologies, also called xe2x80x9crapid prototypingxe2x80x9d or xe2x80x9clayer fabricationxe2x80x9d, to develop prototypes of new products are well known by the market leaders in most manufacturing sectors. First, most of the newer freeform-fabrication machines, such as a stereolithography apparatus (xe2x80x9cSLAxe2x80x9d) and other similar layer fabrication machines, can produce exacting prototype models directly from three, dimensional CAD designs. Second, the prototypes of the new products, made by these freeform-fabrication machines, are commonly used today during the design and development of the new products to determine both the form and the fit of the new product, and, in many instances, allow for functional testing as well. When freeform-fabricated prototypes, made using freeform-fabrication machines like SLA, are used early in the design process, manufacturers are able to preclude most of the design flaws that commonly occur later in the development process when they are much more expensive to correct.
3D Systems of Valencia, Calif., the developers of the stereolithography fabrication process, manufacture the most popular line of freeform-fabrication machines, which utilize stereolithography technology, in use by industry today. In fact, the SLA7000, 3D Systems"" most advanced freeform-fabrication machine, is capable of great accuracy, rivaling even the best CNC machining processes, and has a throughput speed that, for the first time, allows a SLA freeform-fabrication machine to compete as a production system in certain applications.
Other types of freeform-fabrication machines used today to produce freeform-fabricated articles include selective laser sintering (xe2x80x9cSLSxe2x80x9d), fused deposition modeling (xe2x80x9cFDMxe2x80x9d), and laminated object manufacturing (xe2x80x9cLOMxe2x80x9d). Each type of freeform-fabrication machine on the market today produces articles with different characteristics. 3D Systems uses photo-curable polymers that, when cured, resemble many of the popular thermoplastics in use today. Today, the strongest of these photopolymers simulate the physical properties of polypropylene, whereas, the photopolymers that are currently in development are planned to emulate the characteristics of polycarbonate, a tough, resilient thermoplastic used extensively in durable goods manufacturing. Stratysis, the developer of fused deposition modeling (xe2x80x9cFDMxe2x80x9d), offers machines to produce articles in a thermoplastic that emulates the properties of ABS plastic, a material used extensively throughout the automotive and aircraft industries. DTM""s selective laser sintering (xe2x80x9cSLSxe2x80x9d) process uses a variety of waxes, nylons and sintered metals to produce freeform-fabricated prototypes. Helysis, the sole remaining vendor of laminated object manufacturing (xe2x80x9cLOMxe2x80x9d) equipment, produces freeform-fabricated articles using thin sheets of plastics or paper and a cutting laser to produce 3-dimensional articles. While LOM machines are less frequently used today for rapid prototyping, some research is ongoing to employ the process to create articles in carbon fiber and other fibrous materials.
One of the advantages of using a freeform-fabrication machine to fabricate an article is that the interior structure of the article can be easily and cost effectively fabricated in a non-solid, thin-walled or truss style design regardless of the exterior shape of the article. This is important when designing articles that require a high strength to weight ratio. In order to maximize strength while reducing the weight of the article, designers often employ xe2x80x9ctensegrityxe2x80x9d type structural patterns. When an external force is applied to one area of an article having a tensegrity structure, the force is continuously transmitted across all members of the structure. This is the principle upon which many bridges are built. Tensegrity structures are also commonly found in organic structures found in nature such as cells, bone tissue and cartilage. xe2x80x9cHoneycombxe2x80x9d thin-walled type patterns produced in paper, aluminum, and aromatic polyamide cloth are popular for adding thickness to or reinforcing structural composites used in aerospace applications. While not truly tensegrity, the honeycomb geometric shape is popular because of it""s relatively low cost of fabrication. Tensegrity structures offer a much higher potential of weight savings due to their ability to distribute tensional and compressive forces evenly throughout the core. Examples of tensegrity geometric shapes include triangles, pyramids, polygons and geodesic spheres or xe2x80x9cbucky ballsxe2x80x9d to name only a few that are possible.
As mentioned earlier, SLA machines use photo-curable polymers to freeform-fabricate articles. Early photo-curable polymers were not known for their durability or strength. Recent advances in SLA materials hold great promise for a variety of prototyping, tooling and specialized manufacturing applications. The newer resins are more resistant to humidity; possess improved tensile strength and are able to withstand temperatures exceeding 120xc2x0 C.
The freeform-fabricated core composite article comprises a freeform-fabricated core and a plurality of composite skins that are cured under vacuum pressure to produce the freeform-fabricated core composite article. It is sometimes necessary to use an adhesive film to attach the composite skins to the exterior surface of the freeform-fabricated core so that the composite skins will remain in place during the fabrication process.
The freeform-fabricated core is normally formed in the basic shape of the desired composite article from a large variety of materials; such as polymer resin, metal, paper, ceramic/polymer blend, wax and other man-made or naturally produced materials; having a solid exterior surface structure and an interior structure. The interior structure can be completely solid or completely non-solid, consisting of various thin-walled or truss style patterns, or any combination of solid and non-solid patterns in the same freeform-fabricated core. The freeform-fabricated core is fabricated in a freeform-fabrication machine, which substantially builds the freeform-fabricated core on a layer by layer basis that accommodates any geometric design required for the exterior surface structure and the interior structure. The freeform-fabricated core can be formed from several freeform-fabricated core segments, which are then joined together to create the freeform-fabricated core.
The composite skins are thin layers of material that can be either cured or uncured. A cured composite skin can be made from various types of material, but typically comprises fibrous materials reinforced with polymer based binding matrix composite laminates having a fibrous material reinforcement such as seed, bast, quartz, metal, glass, graphite, carbon, E-Glass, polymer based, ceramic or fiberglass fibers disposed in a polymer based binding matrix such as epoxy, vinyl ester or other similar resinous organic material. The uncured composite skin can be made from various types of materials, but typically comprises one or more laminates of uncured preimpregnated fibrous reinforced composite materials having a fibrous material reinforcement such as seed, bast, quartz, metal, glass, graphite, carbon, E-Glass, polymer based, ceramic or fiberglass fibers disposed in a polymer based binding matrix such as epoxy, vinyl ester or other similar resinous organic material. Metal binding matrix and ceramic binding matrix can be substituted for the polymer based binding matrix to provide more specialized composite systems called metal matrix composites (MMCs) and ceramic matrix composites (CMCs), respectively. The uncured composite skins are impregnated with the desired binding matrix prior to mating with the freeform-fabricated core.
When used, the adhesive film holds the composite skins in place against the exterior surface of the core during the fabrication process and promotes the bond between the composite skins and the freeform-fabricated core. The combination of the plurality of composite skins applied to the freeform-fabricated core is called a xe2x80x9ccomposite lay-upxe2x80x9d. The composite lay-up is disposed in a vacuum bag forming a cavity between the vacuum bag and the composite lay-up. As the vacuum pressure is placed on the vacuum bag, it evacuates the cavity to urge the vacuum bag against the composite skins thereby compressing the composite skins against the exterior surface structure of the freeform-fabricated core. The vacuum pressure is held on the vacuum bag, preventing displacement of the composite skins, until the freeform-fabricated core composite lay-up has been cured.
By combining the improved fabrication materials, like improved resins, with the improved throughput speed of the freeform-fabrication machines, like the SLA7000, the potential exists (both economically and technically) for the first time to use a freeform-fabrication machine to produce usable end-use articles without the normal sacrificial production steps. The material properties of articles produced using SLS and FDM machines are improving as well. Moreover, their dimensional accuracy, finish quality and ability to reproduce small intricate features are expected to improve over the next few years to the extent that they too may be used successfully to produce freeform-fabricated core composite articles.
A freeform-fabricated core composite article, as described above, is manufactured by the steps of: designing a freeform-fabricated core for a freeform-fabricated core composite article using a three-dimensional computer aided design (3D CAD) program to create a software file of a three-dimensional design generated by a computer of the freeform-fabricated core where the freeform-fabricated core has an exterior surface structure and an interior structure; modifying the software file of the 3D CAD design of the freeform-fabricated core to select the desired internal structural pattern for the freeform-fabricated core""s interior (i.e.: a tensegrity structural pattern or solid structure) or any combination of solid and non-solid structural patterns as may be required for the particular article""s design; modifying the software file of the 3D CAD design of the freeform-fabricated core to include appropriate interior spaces to accommodate components or other imbedded functionality within the freeform-fabricated composite core; modifying the software file of the 3D CAD design of the freeform-fabricated core to adjust for the plurality of composite skins"" thickness in order to obtain desired dimensions of the freeform-fabricated core composite article after the composite skins are attached to the freeform-fabricated core; exporting the modified software file of the 3D CAD design of the freeform-fabricated core into a machine language software file that is suitable for use with the selected type of freeform-fabrication machine; importing the machine language software file of the freeform-fabricated core into a computer workstation, the computer workstation being of the type which products control signals that controls the selected freeform-fabrication machine, where the freeform-fabrication machine has a build platform situated in a fabrication material storage area which holds fabrication material; processing the machine language software file of the freeform-fabricated core by the computer workstation to create a freeform-fabrication software file that contains a plurality of thin horizontal cross-sectional layers of the freeform-fabricated core along a Z-axis, where the Z-axis is the vertical axis of a three-dimensional Cartesian coordination system; selecting desired orientation for fabricating the freeform-fabricated core on the build platform in the selected freeform-fabrication machine; selecting the type and spacing of support posts used, if required, to separate the core from the fabrication machine build platform during fabrication using the freeform-fabrication software file; fabricating the freeform-fabricated core, with build supports as may be required, with the freeform-fabricated machine and the fabrication material from the freeform-fabrication software file in the computer workstation such that the combination of the freeform-fabrication software file, the computer workstation, the freeform-fabrication machine, and the fabrication material cooperate to fabricate the freeform-fabricated core one thin horizontal cross-sectional layer at a time on the build platform; removing the freeform-fabricated core, with its support posts as may be required, from the build platform in the freeform-fabrication machine; cleaning the freeform-fabricated core with solvent prescribed by the material supplier to remove any contaminants (i.e.: non-polymerized photo-polymers) from the surfaces of the freeform-fabricated core; removing supports from the freeform-fabricated core, if required; curing the freeform-fabricated core by exposing the freeform-fabricated core to an external stimulus (i.e. ultraviolet light or heating) as specified by the freeform-fabrication machine manufacturer to further solidify and harden the freeform-fabricated core; finishing the exterior surface of freeform-fabricated core by hand or machine filling, sanding and polishing to eliminate imperfections, blemishes or excess cured polymer; preparing the plurality of composite skins for mating with the freeform-fabricated core by cutting the uncured composite skins into a predetermined pattern to form a single or multi-layered composite skin to mate with the exterior surface structure of the freeform-fabricated core and by impregnating the composite skins with selected binding matrix material (i.e.: polymer, ceramic, metal, vinyl ester or similar resin compound), or alternatively, using xe2x80x9cpre-impregnated composite skinsxe2x80x9d (uncured composite skins wetted with a binding matrix material and frozen for later use) or using cured composite skins; applying adhesive film between the exterior surfaces of the freeform-fabricated core and the plurality of composite skins, if required; mating the impregnated plurality of composite skins to the exterior surface of the freeform-fabricated core to create a composite lay-up of the freeform-fabricated core composite article; disposing a vacuum bag over the composite lay-up forming a cavity between the vacuum bag and the composite lay-up; evacuating the cavity to urge the vacuum bag against the composite skins, thereby compressing the impregnated composite skin against the exterior surface structure of the freeform-fabricated core to prevent displacement of the composite skins; and curing the composite lay-up wherein the composite lay-up is subjected to external stimulus that solidifies and hardens the composite lay-up to manufacture a freeform-fabricated core composite article.
It is the object of the present invention to provide a method for manufacturing a composite article of high quality and with repeatable results without the use of rigid part specific molds or shaped plugs by creating a freeform-fabricated core using a freeform-fabrication machine and freeform-fabrication material.
It is the object of the present invention to provide a method for manufacturing composite articles having a freeform-fabricated core with a non-solid interior structure to provide a lightweight but strong composite article.
It is another object of the present invention to provide a method for manufacturing a composite article that reduces the steps previously involved to manufacture a composite article.
Another object of the present invention is to provide a method for manufacturing a composite article that reduces the costs of manufacturing the composite article.
It is yet another object of the present invention to provide a method for manufacturing a composite article that reduces the time to manufacture a composite article.
It is yet another object of the present invention to provide a method for manufacturing a composite article that includes imbedding or integrating components or creating additional functionality within the composite article in a cost effective process that also produces high quality composite articles.
Still yet, another object of the present invention is to provide a method for manufacturing a composite article which provides some of the advantages found in the apparatuses and methods of the prior art thereof, while simultaneously overcoming some of the disadvantages normally associated therewith.